1. Field of the Invention
The present invention relates to a connecting structure for connecting between circuit boards. More specifically, the present invention relates to a connecting structure for connecting conductor patterns to each other by bringing them into contact with each other and a method for manufacturing the connecting structure.
2. Background Art
FIGS. 8A to 8C are sectional views showing a connection portion for illustrating main steps of a method for connecting conductor patterns to each other in a conventional circuit board. In FIG. 8A, first circuit board 500 includes first resin base material 502 and convex first conductor patterns 504 formed on the surface of first resin base material 502. Furthermore, second circuit board 506 includes second resin base material 508 and convex second conductor patterns 510 formed on the surface of second resin base material 508. First conductor patterns 504 of first circuit board 500 and second conductor patterns 510 of second circuit board 506 are provided at the same pattern pitch. Note here that on first circuit board 500 and second circuit board 506, in addition to conductor patterns 504 and second conductor patterns 510, further circuit patterns of various shapes are formed, and a functional component such as a semiconductor element and a passive component such as a resistor are mounted although they are not shown.
First conductor pattern 504 and second conductor pattern 510 are electrically and mechanically connected to each other as follows. That is to say, adhesive resin 512 is applied on the surface of second circuit board 506 on which second conductor patterns 510 are formed. As adhesive resin 512, thermosetting resin such as epoxy resin or epoxy-based resin is often used. After adhesive resin 512 is applied, second conductor patterns 510 of second circuit board 506 and first conductor patterns 504 of first circuit board 500 are allowed to face each other and the positions thereof are adjusted.
As shown in FIG. 8B, upper heating pressing plate 514 and lower heating pressing plate 516 press first circuit board 500 and second circuit board 506 in the direction shown by arrow 521, respectively.
As shown in FIG. 8C, in accordance with this pressing, adhesive resin 512 on second conductor pattern 510 is pushed to the side as space between first conductor pattern 504 and second conductor pattern 510 is reduced. When the space is further reduced, almost all the adhesive resin is removed from the space, so that first conductor pattern 504 and second conductor pattern 510 are brought into direct contact with each other. Thus, electrical conduction therebetween is secured. In this state, heating is continued while pressing. When epoxy resin, epoxy-based resin, or the like, is used as adhesive resin 512, adhesive resin 512 is heated at a temperature between 150° C. to 200° C. for 30 seconds to 60 seconds so as to be hardened by heating, and is then cooled down. Thus, electrical and mechanical connection is completed.
In the above-mentioned connecting method, first conductor patterns 504 and second conductor patterns 510 are conducting to each other in a state in which they are just in contact with each other, and they are bonded to each other with adhesive resin 512. In this connecting method, however, when first circuit board 500 and second circuit board 506 are pressed, slipping occurs between first conductor patterns 504 and second conductor patterns 510, so that conductor patterns are often displaced. Furthermore, since it is difficult to securely remove adhesive resin 512 from an interface between first conductor pattern 504 and second conductor pattern 510, a contact area is substantially reduced. Therefore, it is difficult to reduce connection resistance of a contact portion.
In particular, when pattern pitches of first conductor patterns 504 and second conductor patterns 510 are small and the contact area is small, such a displacement or remaining adhesive resin 512 may easily cause defects including short circuit between neighboring conductor patterns and an increase in the connection resistance of the contact portion.
To address such problems, Japanese Patent Unexamined Publication No. 7-74446 discloses the following method. That is to say, it discloses a connecting structure in which an electrical connection portion of a flexible printed wiring board using a thermosetting film as a substrate is formed by soft metal electrode, and this electrode is electrically connected to an electrode portion of the other printed wiring board with adhesive directly and subjected to thermocompression with the use of liquid-type electrically insulating adhesive. In such a method, the soft metal electrode, when it is pressed, is collapsed and extended on the surface of the other electrode by compression bonding to enter fine recessed portions of the other electrode so as to increase the contact area, and therefore, connection resistance can be reduced.
In order to achieve the above-mentioned effect, soft electrode is required to be used as a conductive material. However, as a conductive material generally used as an electrode of flexible printed wiring board, materials such as conductive paste mainly including silver (Ag) or copper (Cu), copper foil or copper plating are often used. In the case of such a material, a large pressing power is required for collapsing and expanding the material. Furthermore, even when such a large pressure power is applied, when fine convex and concave portions exist in the electrode portion, it is difficult to completely remove all the liquid electrically insulating adhesive existing in the concave portion. Therefore, this electrically insulating adhesive may remain in a part of the electrode surface, making it difficult to satisfactorily reduce the connection resistance of the contact portion. As the connecting pitch becomes smaller and the contact area is accordingly reduced, the effect caused by the fact that this connection resistance cannot be reduced is remarkable.
The object of the present invention is to provide a connecting structure of a circuit board in which no adhesive resin is provided between conductor patterns of a connection portion and even when conductor patterns are arranged with a fine pitch, connection resistance is small and short circuiting between neighboring conductor patterns does not occur.